One of my goals is to get more suspension travel, specifically more bump travel. I plan to get most of that by raising the car, but I also wanted to use a shorter bump stop both front and rear.
In the rear I decided to use one of the Z4 rear bump stops, because it's rumored that Dinan is using it in their M3 lowering kit. The part I chose was P/N 33 53 6 770 350, which includes the bump stop and protection tube. (The bump stop doesn't fit the E90 stock tube.)
It turns out this bump stop is 2.5" long, making it 0.375" shorter than the ZSP stock bump stop. Here's a picture of the two side by side:
As you can see it's not much shorter. In the future I might choose a shorter one, but a conservative change seems best for now. Also, Koni's bump stops don't seem to be used with the stock dust tubes, and I'd have to find a solution for that.